Characterization of a nitrogen-fixing bacterial strain from the roots of Digitaria sanguinalis

Nitrogen-fixing (acetylene redution) activity and population of aerobic heterotrophic nitrogen-fixing bacteria associated with wetland rice

Nitrogen-fixing activity associated with different wetland rice varieties was measured at various growth stages by an in situ acetylene reduction method after the activities of blue-green algae (cyanobacteria) in the flood water and on the lower portion of the rice stem were eliminated. Nitrogen-fixing activities associated with rice varieties differed with plant growth stages. The activities increased with plant age, and the maximum was about at heading stage. The nitrogen fixed during the whole cropping period was estimated at 5.9 kg of N per ha for variety IR26 (7 days) and 4.8 kg of N per ha for variety IR36 (95 days). The population of aerobic heterotrophic N(2)-fixing bacteria associated with rice roots and stems was determined by the most-probable-number method, using semisolid glucose-yeast extract and semisolid malate-yeast extract media. The addition of yeast extract to the glucose medium increased the number and activity of aerobic heterotrophic N(2)-fixing bacteria. The glucose-yeast extract medium gave higher counts of aerobic N(2)-fixing bacteria associated with rice roots than did the malate-yeast extract medium, on which Spirillum-like bacteria were usually observed. The lower portion of the rice stem was also inhabited by N(2)-fixing bacteria and was an active site of N(2) fixation.

Isolation and Characterization of Dinitrogen-Fixing Bacteria from the Rhizosphere of Temperate Cereals and Forage Grasses

N 2 (C 2 H 2 )-fixing bacteria were isolated from the rhizosphere of various cereals and forage grasses grown in the greenhouse and from the rhizosphere of field-grown wheat in Sweden. All 46 isolates from the greenhouse plants lost their nitrogenase activity during purification. By imposing a stronger selection pressure, we obtained pure isolates with nitrogenase activity from field-grown wheat. Some isolates were identified as Enterobacter agglomerans and Bacillus polymyxa , but several bacteria of uncertain taxonomy also occurred. One of the isolates grew and reduced acetylene only in the presence of other bacteria or certain vitamins. Species of Azotobacter or Azospirillum could not be isolated from the rhizosphere of any of the cereals and forage grasses studied.

Root-Associated N 2 Fixation (Acetylene Reduction) by Enterobacteriaceae and Azospirillum Strains in Cold-Climate Spodosols

N 2 fixation by bacteria in associative symbiosis with washed roots of 13 Poaceae and 8 other noncultivated plant species in Finland was demonstrated by the acetylene reduction method. The roots most active in C 2 H 2 reduction were those of Agrostis stolonifera, Calamagrostis lanceolata, Elytrigia repens , and Phalaris arundinacea , which produced 538 to 1,510 nmol of C 2 H 4 ·g −1 (dry weight)· h −1 when incubated at pO 2 0.04 with sucrose (pH 6.5), and 70 to 269 nmol of C 2 H 4 · g −1 (dry weight)·h −1 without an added energy source and unbuffered. Azospirillum lipferum, Enterobacter agglomerans, Klebsiella pneumoniae , and a Pseudomonas sp. were the acetylene-reducing organisms isolated. The results demonstrate the presence of N 2 -fixing organisms in associative symbiosis with plant roots found in a northern climatic region in acidic soils ranging down to pH 4.0.